The Mostly Mozart festival began 49 years ago and, for better or worse, over the ensuing decades has strayed from its namesake and inched toward the contemporary.

This year’s edition opened with an (almost) all-Mozart program played by the wonderful resident orchestra Wednesday night at Avery Fisher Hall under the direction of Louis Langreé joined by celebrated pianist Emanuel Ax and, making her festival debut, soprano Erin Morley.

The Mostly Mozart Festival Orchestra played the brief overture to Mozart’s operetta DerSchauspieldirektor. That work is a comedy about the egos of opera singers and producers and the humor showed through Wednesday night with glissando after glissando vying for attention.

That made a sprightly intro for Ax performing the Piano Concerto No. 14 in E-flat Major. Ax quickly claimed ground within the orchestra, playing the unaccompanied trills of the waltz-time first movement with aplomb. The strings returned to gently encircle him and they continued with a closely measured pace, Ax at times tugging the reins and slowing, slightly, the tempo.

The slow movement gave Ax another unaccompanied passage before delving into the score’s despondency, the strings beautifully singing his accompaniment. The pianist’s encore of Schumann’ Fantasiestücke was the only deviation from the Mozartian norm.

Resplendent in red, Morley opened the second half with two arias commissioned from Mozart by his sister-in-law Aloysia Lange for her performance of Pasquale Anfossi’s Il curioso discreto in 1783. Slowly swaying to the music, Morley gave an almost remorseful reading of “Vorrei spiegarvi, oh Dio” a song of a woman rejecting the advances of a suitor because she is already engaged, almost anchored by the weight of the emotion in the aria.

“No, che non sei capace,” found the same character responding to charges of adultery, and Morley deftly delivered the extreme arpeggios with a restrained anger.

From humoresque to heartache, the program ran a wide emotional gamut. The orchestra had sounded fantastic the whole evening, but were at their finest in the closer, Mozart’s Symphony No. 34.

The players negotiated the steep dynamics and shifting tempos with apparent ease. Written at a time when the 22-year-old composer was mourning the death of his mother, and suffering from romantic and career frustrations, the score is fairly bursting with thematic riches in all four movements.

The orchestra delivered the goods with remarkable clarity. Langreé nearly danced his way through the ripples and rolls of the second movement. The third movement seems to peter out, with the conductor falling to a slouch before Langreé brightly picked up the microphone to explain “Mozart suddenly stopped writing in the middle of this minuet and then jumped to the finale.” After that comical, slightly awkward moment, the finale made a rousing close to the concert and the evening. And where, if not Mostly Mozart, will the great man’s asides and abandoned ideas be heard?